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Is it expected to see transition from laminar to turbulent BL at low Reynolds numbers?
I have used XFOIL to do the following analysis:

NACA0018 airfoil.
Reynolds number $2\times 10^4$ (low Reynolds number).
Critical Amplification factor, $N_\mathrm{crit} = 3.54$ (corresponding to a freestream turbulence level of 0.7%). I have used the formula from this thread.

When I run the analysis at $\alpha = 5^\circ$, I get the following results:

However, I am confused as I don't understand how to interpret the results:

Is the plateau in pressure coefficient an indication of transition from laminar-to-turbulent boundary layer? if so, I feel it's wrong since the freestream Reynolds number is very low ($2\times 10^4)$ and I do not expect transition at such low Re numbers (I can't find the source, but I read somwhere that the transition from laminar-to-turbulent is expected for $Re \geq 7\times 10^4$ ).

Is the plateau in pressure coefficient an indication of transition from laminar-to-turbulent boundary layer?

Yes. The plateau is actually the separation bubble and transition happens at the rear end of it, shortly before the steep pressure increase made possible by the now turbulent boundary layer. Check the Xtr output of XFOIL! This you will get for example by plotting the boundary layer details, or when writing a sweep of points to a file.

I read somwhere that the transition from laminar-to-turbulent is expected for ≥7×10⁴

That depends on the history of the boundary layer and the local pressure gradient. See it this way: A positive pressure gradient means that the flow is decelerated. The initially small disturbances (flow speeds orthogonal to the actual direction of flow) will grow relatively larger when the speed in the direction of flow sinks. This allows transition already at Re well below 10⁵, and with your very low Ncrit even at 10⁴. Normally, Ncrit should be around 9.
Besides, your Reynolds number is very low, even for XFOIL. Did you have a chance to compare them with actual measurements?

Thank you for your answer. The reason I have a doubt about the LSB is the following paper (See figure 2):  J. Winslow, H. Otsuka, B. Govindarajan, and I. Chopra, “Basic understanding of airfoil characteristics at low Reynolds numbers (104–105),” J. Aircr., vol. 55, no. 3, pp. 1050–1061, 2018.


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What is the bandwidth of a 1090ES ADS-B signal?
I cannot find an official, trustworthy source on the bandwidth of the 1090 Mode S extended squitter ADS-B signal. It is not even listed in DO-260C standard. I've seen ranges from 50 kHz to 11 MHz (https://www.komachine.com/en/companies/ap-satellite/products/53719-ADS-B-(Automatic-Dependent-Surveillance-Broadcasting)-System). SigID Wiki claims 2 MHz in the US (https://www.sigidwiki.com/wiki/Automatic_Dependent_Surveillance-Broadcast_(ADS-B)), but I've also seen 1.3 MHz.

You have to define and be consistent about what you mean by "bandwidth." Theoretically, the absolute bandwidth of most signals are infinite. There are various definitions: -3 dB, -10 dB, -20 dB, -40 dB, and non-linear definitions (spectral channel masks). The latter is what most standards, including ICAO defines.

@user71659 OK, I agree with you. Most times we use 3dB bandwidth, but you indicate a spectral mask is used in this case/purpose. How does that help if I do not see the signal or the mask in any documentation.

The bandwidth is not specified in the ADS-B MOPS as the 1090ES transmitter is defined in the Mode S MOPS.
Here is the graphical description of the required transmitter bandwidth from the ICAO SARPS:

The transmitted data is within the 2.6 MHz, -3dB portion.
The raw data rate of the extended squitter is 1 Mb/sec. The data is encoded using pulse position modulation (PPM). In PPM encoding, there is a 1 microsec slot for each bit with a 1/2 microsec pulse located in either the first or second half of the slot to indicate a 1 or 0, respectively. As the pulses are a 1/2 microsec in width, the necessary bandwidth is 2 MHz.

Yep, you are correct. The spectral mask information is located in DO-181F. Thanks.

As shown by the SigID Wiki page you linked, 1090 MHz ADS-B (the international version) has a bandwidth of 2 MHz. The 1.3 MHz version operates on 928 MHz and is only used in the United States.

UAT is not the same as 1090ES ADS-B.

UAT carries ADS-R, TIS-D and FIS(weather and notam).  Of those only TIS is available by relay on 1090ES.

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Is it permitted to land amphibian plane in the ocean on Californian coast?
Given the length of the Californian coast, I was surprised to find so few seaplane services.  I would expect air taxis between Long Beach and the coast, for example.
Is it permitted to land seaplanes along the coast at, say, Zuma Beach geo://34.016286,-118.820402 and do whatever local boats do for mooring?

In California, for example, you will not need to obtain certification of either a landing area or mooring area. Seaplane Pilots Association

There appear to be very few seaplane bases in California. Seaplanebase only lists five.

It is not clear from the title precisely what you are asking. The question in the text seems to be "Is it permitted to land seaplanes along the coast [of California]". Perhaps you could amend the title accordingly.

@Fiddlesticks thanks for suggestion, done

Where would seaplanes from these bases go as a destination?

Related

@GdD I was imagining charter flights from Long Beach, which has a great deal of GA, and ocean-front properties in Malibu, which can be reached directly from the beach.

There's a problem with that, seaplanes can only be operated in relatively calm sea-states, they need sheltered areas, and even then you'd have plenty of times when they just couldn't be flown. Most seaplanes fly from rivers and lakes, not the ocean, where they do it's with limitations.

@GdD Ah yes I can see that a place famous for surf is likely to be terrible for seaplanes.  But I'm really after knowledge of California permissions, not how often the conditions are suitable.

@GdD: And most of the California coast is often foggy, meaning you'd need instrument approaches & departures.

In a free country such as the US, that which is not specifically prohibited is permitted.  Float planes fly into remote lakes all the time without requiring special government permission.  I don't think you will find anything in the federal regulations that prohibit water landings off the California coast, but there may be wildlife refuges to consider or local noise abatement concerns.

Seaplane air taxis only work where the logistics situation and market can support a business case for that kind of service.  It's an expensive way to travel, so there has to be some kind of benefit that makes it worth the cost to the traveler.
This works in the Seattle area and the Lower Mainland of British Columbia where you have large city centers with nearby destination locations that require over-water travel to get to, and business travelers willing to pay extra for fast waterfront-to-waterfront travel.
Vancouver BC has a thriving air taxi market because people want to get across to Vancouver Island quickly, especially Victoria BC, the provincial capital, and the business traveler demand supports seaplane operations up to Twin Otter size.
California? Way cheaper to drive, or go taxi-airport-taxi.  In Vancouver BC or Seattle, a seaplane air taxi service is competing with ferry boat service so it has a serious leg up that makes the ticket price worthwhile. Not so much down south on a contiguous coastline where there are both land and air alternatives.
So bottom line is, there's no money to be made doing it.

Respectfully, that appears to be an opinion of the economics.  Typical ocean-front properties in that area are $10m+, many of their owners are privacy and comfort sensitive.  Given that you could fly directly to their properties, I'm surprised not to see it.  Hence my question wasn't about whether it was economic, but whether it was permitted by the various authorities.

No worries.  I inferred that you were wondering why there aren't any air services on the coast, not just the strict legalities.  In any case when I was a bush pilot the air service I worked for had a number of very wealthy customers and they were generally surprisingly cheap people lol.

@jonathanjo, While you point out you are asking about regulations only, the wonderment you express about why there isn't more seaplane service along the coast naturally provokes answers as to why it would be economically and operational unfeasable.  The Pacific Northwest has lots of protected waterways, Zuma beach, not so much...

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How do I reinforce a tubular spar that has been drilled vertically for an attachment bolt?
Previously posted a question about lift strut attachment to tubular wing spar with a  bolt vertically through the spars.  See drawing below.

I tried using Bruhn's analysis book to formulate a a reinforcement to the tension and compression areas of the spar. I'm not an engineer, and need guidance.
I don't know enough or what formula to calculate the loss of strength due to the 3/8" bolt hole in the tension and compression surfaces of the spar.
I can provide the following data for calcualtions if that's helpful, the site does say to provide as much inofrmation as possible.
The eyebolt the strut attaches to is 101 in from the root, and the moment of lift at the strut attachment point from the outboard portion of the wing was calculated to be 10015.  WIth an OD of 2.00 and and ID of 1.75, the moment of inertia at the strut attach was .303.  This gave a stress developed at this point as 33,052.  Increasing the OD to 2.125  as in solution B increases the moment of inertia to 0.533, and reduces stress to 29940.   and OD to 2.25 in solution A to increases moment of inertia to 0.742 and reduces stress to 26994.  These were calculated with a safety factor of 1.5.
Does solution A create stress risers at the ends of the 2.25in external sleeve?   Does the solution B lack sufficient compensation for the loss of material?

@robert DiGiovanni... in this added drawing, the middle presentation shows the original design, the single bolt horizontal through two straps which connect to the lift strut.

I feel we are veering off of Aviation, and into structural engineering.

@ToddMcLauchlin I'll repeat a comment I added to your other post... you need to find a book on the design of tubes as beam spars. You need to know how to do the stress analysis to select and size the material, then when you have a design you have to build it and do a static load test on it (set it up upside down on a test fixture and pile on the sand bags) up to the intended load limit to validate the analysis. That's if you actually intend to bet your life on the structure in the air. If you don't plan to do all that, don't bother. Just build a Goat from the free plans available on-line.

Well, John K, I'm trying to avoid having to REPEATEDLY design something only to have it fail repeatedly.  That's the point of all this...guidance, like, yeah you're on the right track, or no, your solution fails to compensate for....   but it that's too much for you and your friends here, then you're really not all that helpful, are you?  I have a book...I did some calcs...  isn't the purpose of this site to globally share wisdom and experience?  I really don't think your condescension is at all helpful.  But thanks for the tip on the goat.... I'll send it to you when I get it fiinished.

The sleeve in the middle drawing has potential.  As always, it's strength vs weight penalty.  When in doubt, overdesign (or get good advice).

@ToddMcLauchlin I wasn't trying to offend, just pointing out reality in the event you are intending to fly your design.  I'm not actually a stress guy, just someone with some practical knowledge and experience, and I don't know if there actually are any on this particular forum that can give you that sort of detailed guidance.  You'll probably have better luck on a structural engineering forum since the questions you have could really be applied to anything using tubes as cantilevered beams.

I appreciate THAT piece of guidance.  Just for background, it isn't my design, it is someone else's that has been successfully flown without any problems.  The particular spar in question came from a partially completed unit, and this is a modification I am questioning.  I did download Bruhn's stress analysis text, and given that it is 966 pages,  I am just a little bit lost in trying to figure this out.  True, I could just buy another set of tubing and fabricate the spar to it's original design.....but what fun and education would THAT be?  You comment about life is well heeded, however.  thx

@Todd McLauchlin could you please include a diagram of the original design in your question?

Robert  DiGiovanni, i had this in a previous question, and if they'll allow it, I 've added a drawing as an edit...we'll see if they allow it to move forward.

@Todd McLauchlin thanks so much.  Gotta go with John K on this one.  Unless you test a mod to failure (the one with the sleeve), you just won't know.


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How long can a foreign registry airplane stay in the US before having to change the register to N or having to leave the country?
I want to take my Seawind (experimental) registry of Nederland's to US in a Container for one or two years to fly around the US.
If I take it flying, is compulsory to have ADS-B out?

Welcome to Aviation.SE! The title of your question does not match the body. Can you clarify what you're asking about?

The US does not have a restriction on how long a foreign-registered aircraft can remain in the US. Other countries do, but the US does not. There are some restrictions, though, listed in 14 CFR § 91.711.
§ 91.711 Special rules for foreign civil aircraft.
(a) General. In addition to the other applicable regulations of this part, each person operating a foreign civil aircraft within the United States shall comply with this section.
(b) VFR. No person may conduct VFR operations which require two-way radio communications under this part unless at least one crewmember of that aircraft is able to conduct two-way radio communications in the English language and is on duty during that operation.
(c) IFR. No person may operate a foreign civil aircraft under IFR unless -
(1) That aircraft is equipped with -
(i) Radio equipment allowing two-way radio communication with ATC when it is operated in controlled airspace; and
(ii) Navigation equipment suitable for the route to be flown.
(2) Each person piloting the aircraft -
(i) Holds a current United States instrument rating or is authorized by his foreign airman certificate to pilot under IFR; and
(ii) Is thoroughly familiar with the United States en route, holding, and letdown procedures; and
(3) At least one crewmember of that aircraft is able to conduct two-way radiotelephone communications in the English language and that crewmember is on duty while the aircraft is approaching, operating within, or leaving the United States.
(d) Over water. Each person operating a foreign civil aircraft over water off the shores of the United States shall give flight notification or file a flight plan in accordance with the Supplementary Procedures for the ICAO region concerned.
(e) Flight at and above FL 240. If VOR navigation equipment is required under paragraph (c)(1)(ii) of this section, no person may operate a foreign civil aircraft within the 50 States and the District of Columbia at or above FL 240, unless the aircraft is equipped with approved DME or a suitable RNAV system. When the DME or RNAV system required by this paragraph fails at and above FL 240, the pilot in command of the aircraft must notify ATC immediately and may then continue operations at and above FL 240 to the next airport of intended landing where repairs or replacement of the equipment can be made. A foreign civil aircraft may be operated within the 50 States and the District of Columbia at or above FL 240 without DME or an RNAV system when operated for the following purposes, and ATC is notified before each takeoff:
(1) Ferry flights to and from a place in the United States where repairs or alterations are to be made.
(2) Ferry flights to a new country of registry.
(3) Flight of a new aircraft of U.S. manufacture for the purpose of -
(i) Flight testing the aircraft;
(ii) Training foreign flight crews in the operation of the aircraft; or
(iii) Ferrying the aircraft for export delivery outside the United States.
(4) Ferry, demonstration, and test flight of an aircraft brought to the United States for the purpose of demonstration or testing the whole or any part thereof.
[Doc. No. 18834, 54 FR 34320, Aug. 18, 1989, as amended by Amdt. 91-227, 56 FR 65661, Dec. 17, 1991; Amdt. 91-296, 72 FR 31679, June 7, 2007]


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What are these strips on HAL Tejas?

What are these strips on and below the tail of this HAL Tejas?

For more info on formation lights (not, imho, a duplicate): https://aviation.stackexchange.com/q/32026/7532

Fin and Tail formation lights.

Look up slime lights. When lit, they look like slimy green, hence the name. They are used for flying formation at night when it isn't possible to use visual clues such as wingtips, missiles, intakes, etc. Basically for dark/night visual pickup, formation keeping. They are for night operations and have visible and infrared modes (for use with NODs).


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How high can an airplane be spotted with eyesight?
How high can an airplane be spotted with eyesight? Suppose a plane has a wingspan of 15 m then at what distance it will appear as a faint dot?
What is the correct way to calculate this?

Do weather conditions at the time create contrails ?  What colour is the underside of the plane?  Does it have running lights?  Is it day or night?

@Criggie  Assuming it's day time and there are no contrails.

I'm a pilot, and I can have Air Traffic Control tell me, and see it on my ADS-B display, about a plane at some distance and altitude, so I know right where to look. White plane moving over a dart tree background are easiest to spot (the movement really helps), and given all that, it can be really hard to find that plane when it 3+ miles away (~15,000 feet).  That would be near ideal conditions. Get a bright background behind and the plane above, and it gets much harder.  Large commercial planes are a lot easier to spot, especially when they are east of you and the sun is west. Or west/east.

I can recall watching satellites pass overhead.

Is this really an aviation question? If I asked this question about cars, would that make it on topic for the automotive SE?

@Acccumulation put your question on automotive SE and count the upvotes. You will come to know if it is on topic or not.

It's been a long time, but I think that in my youth I saw a T-38 at 25,000 feet.

I think the question "Suppose a plane has a wingspan of 15 m then at what distance it will appear as a faint dot? What is the correct way to calculate this?" is  ambiguous as to whether it is asking about the threshold between being able to resolve some detail vs appearing as faint dot, or the threshold between appearing as a faint dot vs not appearing at all.  Perhaps too late to edit now as edits are not supposed to invalidate existing answers and at least one answer has assumed each interpretation.

Just to note, you ask how high a plane can be spotted, but higher flying planes typically tend to be larger (e.g. jet planes versus prop planes).  15 meters is probably at the lower end of what you'll see that high, as it's about the wingspan of small business jets.

@CrossRoads What is a "dart tree background"?

That was supposed to be dark tree background. Vs say, a lighter background such as snow, or perhaps the ocean.

Note: There is some confusion in the comments about this answer. What I am answering here is the question:

Suppose a plane has a wingspan of 15 m then at what distance it will appear as a faint dot? What is the correct way to calculate this?

This does not imply that you cannot see the airplane at a greater distance any more. It just means you cannot resolve it any more, i.e. it will appear as a faint dot.

This can be estimated with the Rayleigh criterion. The angular resolution of the human eye is given by
$$ \theta = 1.22 \frac{\lambda}{D} $$
where $ \lambda $ is the wavelength of the light and $ D $ is the pupil diameter. Using typical values of $ \lambda = 550 \, \mathrm{nm} $ (highest sensitivity of the human eye) and $ D = 3 \, \mathrm{mm} $ during the day, we get a resolution of
$$ \theta \approx 2.24 \times 10^{-4} \, \mathrm{rad} \approx 0.013^\circ $$
If the aircraft is $ w = 15 \, \mathrm{m} $ wide, the distance at which this corresponds to this angle is given by
$$ d = \frac{w}{\tan \theta} \approx \frac{w}{\theta} \approx 67.1 \, \mathrm{km} $$
Most planes only fly up to FL410, which is about 12.5 km, so you could always distinguish them from a faint point with the naked eye. As Zeiss Ikon pointed out in the comments, the actual resolution of your eyes may be worse because your eyesight is probably not perfect. This is however an upper limit on the resolution.

Based on my experience, human vision doesn't always live up to the Rayleigh criterion.  That said, I've spotted the tiny arrowhead at the leading end of a contrail on many occasions.

@ZeissIkon That's true, this is the limitation from physics and therefore an upper limit. Human vision can definitely be worse, but how much worse will depend on the particular person's eyesight.

This seems like at best a partial answer, since it's perfectly possible to see smaller objects like satellites at a considerably greater distance, depending on how they're illuminated.  Likewise, it's common to see planes around sunset (and presumably sunrise, if you're up that early), when the angle is such that sunlight reflects off it.

@jamesqf This is answering how far they can be that you can still resolve them, i.e. see them as something other than a dot. You can see satellites but only as dots.

The Rayleigh criterion only applies to resolving an object as having a shape.  With sufficient contrast (eg. sunlit satellite against night sky), you can see things at a much smaller angular resolution.

And stars are pretty far away... :)

Probably worth noting that that this only applies in ideal (i.e., vacuum) conditions. Even perfectly clear weather will add a little bit of attenuation from the air, and wind could fuzz that some more.

The question seems to be asking for the maximum altitude at which a plane would be visible as a dot. This answers an interesting, but different question: the maximum altitude at which a plane would be resolvable as being plane-shaped.

@Skyler: Yes, that's my point.  You're just addressing resolving power, but the question asks how far away you can see them.  If you just see sun reflecting off the fuselage, or a bright landing light at night, you're still seeing the plane even if you can't resolve it.  Likewise, if it's against a similarly-colored background, you might not be able to see it, even if you could easily resolve it against a different background.

"Most planes only fly up to FL410", yes, by the time they're at 67km it's a very special plane. The SR-71 didn't even reach half of that.

This is like answering a question about what the maximum speed of a Cessna is by saying that according to the theory of relativity, it's impossible for it to travel faster than the speed of light. And concluding that there's no problem taking a Cessna supersonic, because that's well below the speed of light.

You can spot an airplane at any distance provided it's lit appropriately. Although when we get to interstellar distances, it becomes prohibitively difficult to resolve it from its illuminating object. An interesting version of the question would be at what distance you can detect an airplane against the day sky - as the plane moves higher, there becomes more air before it and the observer, and less air behind it to be occluded.

Re your recent edit, I think the question "Suppose a plane has a wingspan of 15 m then at what distance it will appear as a faint dot? What is the correct way to calculate this?" is  ambiguous as to whether it is asking about the threshold between being able to resolve some detail vs appearing as faint dot, or the threshold between appearing as a faint dot vs not appearing at all.

Under ideal viewing conditions (a background of thin high clouds, and an optimal color scheme on the aircraft), a radio-controlled aircraft with a 2-meter wingspan and a high aspect ratio can be seen clearly enough to actually maintain direct control unassisted by any electronic stabilization or autonomous navigation capability at 4000 feet (1.22 km) above the ground.  (Source-- own experiments with well-calibrated onboard telemetry.)  An aircraft with a 15-meter wingspan would have the same apparent wingspan at 30,000 feet (9.14 km) above the ground.
Note that in some circumstances it is possible to maintain control of an aircraft in this manner even if it appears to be little more than a tiny speck with few distinguishable features, because the aircraft has good intrinsic aerodynamic stability, and the aircraft's approximate heading is revealed by its direction of travel.  Still, if the only criteria that is that the aircraft is perceptible at all, the corresponding altitude would be much higher.
After first being spotted with binoculars, Mississippi kites have been observed with the naked eye by observers with no more than 20/20 vision, as the birds passed in and out of the bases of cumulus clouds that were verified by consulting nearby airport ATIS/ AWOS broadcasts to be at least 4000 feet (1.22 km) above the ground.  (Source-- own observations.)  (The birds were not identifiable to species with the naked eye at this distance and may have appeared as little more than tiny specks; I don't have detailed notes accessable to refresh my memory at this time.)  The average wingspan of a Mississippi kite is about 80 cm.  The corresponding altitude for an aircraft with a 15-meter wingspan would be 75,000 feet (22.9 km).
Next time you watch an airliner fly high overhead, ask yourself "what is the smallest part of that aircraft that I can actually discern", and then do the math.  You'll likely come up with a number that is at least double the aircraft's cruising altitude, and possibly much higher.
Note that the question does not appear to actually be asking at what altitude an aircraft can be visibly discerned to be something other than a tiny unknown speck.  Rather, it appears to be asking at what altitude an aircraft can be visibly discerned at all.
However, since the question mentioned "spotting", keep in mind that in most daytime viewing conditions it is much more difficult to initially detect an object than to keep it in view once it is detected.  Even if the object has been spotted and its approximate position is therefore known, it can be very difficult to re-acquire it if visual contact is lost.  When scanning an empty blue sky, the eye tends to focus much closer than would be appropriate for detecting distant objects.  Other distant features (e.g. contrails, high clouds with visible texture, the moon, etc) can be very helpful as they provide something for the eye to focus on.  So the ultimate answer to the question is "the probability of spotting (within a given time interval) a given aircraft flying at a given altitude depends tremendously on the prevailing conditions such as background features and lighting conditions, and also on the aircraft's color scheme."  With light-colored aircraft, brown polarized sunglasses can be extremely helpful, especially in the haze-free conditions that tend to prevail at higher ground elevations, as they can dramatically darken the blue sky.

While piloting similar r/c sailplanes at rather lower altitudes but still "specked out" as we say, passersby have often wondered at the gizmo in my hands and my skyward gaze.  More often than not, even with careful cues "that dragon-shaped cloud, two legs down and to the left, here, I'll put her in a spin so the wings glint," they can't spot it.  You have to know where to look.  Even in full scale aviation, spotting another airplane heading at you at your own altitude is terrifyingly difficult even just a few seconds away.

20/20 vision is about 1/3000 radians. So an object that is 15m wide can be discerned about 3000*15m = 45km away. This is only a rough estimate, since there are a multitude of factors such as how good someone's eyesight is, how clear the air is, etc.

If you apply that formula to the night sky, you'll conclude that the stars are invisible.

@Mark We are unable to resolve the stars. That's quite different from not being able to see them.


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Radome replacement process for radome damaged by bird strike
Suppose a flight has a bird strike which requires the radome to be replaced. The aircraft is grounded at Chennai (MAA), India. What is the process of its replacement? Where will the new randome come from? Who will conduct the repair? The aircraft is A320 neo.
Line maintenance facility is available at MAA but no base maintenance. Base maintenance facility/MRO/workshop available at BLR (350km) and HYD (650km). Will the airline decide to fly it there or bring radome/spare parts to MAA?

An answer here would require knowledge of where the un-named airline has replacement radomes available, which is probably beyond general internet knowledge. Likewise, decision makers at the airline can choose among options like using own personnel, contract personnel, Airbus mechanics (if things are complicated enough), or flying it to someplace with heavy maintenance available, but they'd be working from knowledge like the extent of the damage (not given in the post) and availability of personnel (not generally knowable across the internet).

I have experience in this area (not with Airbus). The airline will do whatever is (1) fastest and/or (2) cheapest.
If the airplane can be ferried, say by applying speed tape over the damage and allowing a ferry flight with, say, a speed/weather limitation to BLR/HYD, under an Airbus tech support disposition, it will probably do that, and ferry the plane to BLR or HYD, whichever base has a radome.
The airline will contact Airbus's 24/7 tech support with the details of the damage with a request for evaluation for feasibility to ferry, and if so, a repair authorization document signed by an Airbus engineer with detailed instructions (if it's just a crack with maybe a little deformation, that might work; if it's a gaping hole, probably not).
If the airplane can't be ferried, the airline will either contract with a 3rd party organization on site at MAA, or send its own maintenance team to MAA and ship a spare radome there to meet the team.
The radome will come from the airline's own spares inventory if it has one in stock, or it will order one AOG (Aircraft On Ground) from Airbus Spares, who will ship a unit from the nearest spares depot in the Airbus spares network via whatever shipper it uses for AOG spares (like DHL or whoever).
If the airline has a spare radome on hand and it has other flights going to MAA, and there is room in a cargo hold of one of them, it might send it along that way with the repair crew. Or, if that doesn't work and time is critical, it might even charter an airplane that can haul its spare radome and crew to MAA.
You do what you have to do, and whether you prioritize cost or speed will determine the actions you take, and that will depend on how long you can afford to have the plane grounded. For airlines that don't keep spare dispatchable aircraft, speed is normally of the essence because of the cost of follow-on cancelled flights, so they will do whatever it takes to get the plane back in the air ASAP.


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What is the cost of electricity generated in the E175-E1?
I'm comparing two types of cabin illumination, and I'm having a difficult time to estimate the cost that will be saved by utilizing lights with a lower power consumption.
What are the costs of electricity on board an airplane? Basically, I am looking for the cost of a kWh of electrical power generated by the engine, during an average travel of about two hours.
I edit to any kind of airplane, because some aircraft are easy to find data. Some information I found already:

In round numbers, a 737 will burn 5,000 pounds (750 gallons) an hour. (https://www.usatoday.com/story/travel/columnist/cox/2014/10/12/boeing-737/17036931/)
An A320's two alternators can produce 150 kilowatts (kW) and needs only 50 kW in total. (https://www.safran-electrical-power.com/media/20151109_more-electric-aircraft-how-does-electricity-work-plane)
The cost of fuel is approximately $1/gallon (https://www.airlines.org/argus-us-jet-fuel-index/)

If somebody have any idea, I would be very grateful.

PSFC of typical turbo shaft is about a little less than pound per kWh.

Do you assume that the engine is running for no purpose other than turning the generator? So the entire cost of the fuel burned is divided into the electricity produced? Or should some of that cost be accounted to things like, moving people over long distances at high speeds?

Thank you for the comments, I edited the question with more information.

If I understand correctly, you're looking at the fractional cost of running the engine over a 2 hour flight. One fraction, very large, is for generating the thrust necessary to keep the plane in the air and moving toward its destination, another fraction is for bleed air to run the PAKS to keep the cabin comfortable, and another fraction is to generate electricity to run all the on-board electronics. It's this 3rd fraction that you're interested in, particularly the cost to run the cabin lights, not the avionics, radios, etc. Is this correct?

Do you assume costs are accounted in extra fuel burnt for electric generation (i.e. a quantity of fuel)?

I edited to any kind of aircraft and some informations I found. That's exactly it. If I have an approximately number of the cost of the fuel for each day in normal operation, what is the percentage that can be used in the ilumination? What would be the cost of the kWh? Thank you all in advance.


text:
How to create a simple forces diagram using only lift and drag forces?
I would like to create a simple forces diagram (like a sketch). The lift produced by the wing depends upon to the distance from the the longitudinal axis, but, in order to make it easier, I’m taking the lift as a constant in the whole wing. Here is the question, I need to calculate the moment produced by the wing taking only the drag and lift forces by the wings, what distance should I take?
Although I think the answer will be a percentage of the wingspan, here is some data if needed.

Wingspan: 1m
AR: 11
Wing configuration: tapered
Root chord: 0.17m
Tío chord: 0.12m

Sketch

Since I have the drag and lift forces and Cp, I need to calculate the distance from the x axis to this force application points. How can I estimate those distances?

You may add drawing of some solutions you thought of and explain what you think about those solution, it would add clarity. For example, it would explicit which moment you are talking about (is it pitch moment, in which case your drawing will show sides of the aircraft, or is it roll moment, in which case your drawing will show either back or front of the aircraft)

In order to determine the moment, you need to find the center of pressure for lift, and the center of drag for drag.  These distances from the attachment to the fuselage x the force gives you your moments.  These need not be perfectly exact, but close enough to provide a point for realistic overdesign, such as +/- 4G.  Knowledge of $Lift$ $distribution$ is helpful here as well.  Like Manu H says, start with few drawings, take a picture, and post it here for our review.

I'm using XFLR 5 to calculate Cp and something similar to the center of drag.

You diagram is weird. for example, does the lift apply on the middle of the left wing or way below the aircraft? Why is your drag vector beginning way in front of any part of the aircraft? See for examples the diagram on this webpage to see how it compares to yours.


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What is the produced Drag by a Jet Engine on an airliner?
Are there certain estimation formulas to calculate the drag created by a Jet engine mounted above the fuselage, as shown?
Digital DATCOM seems not to estimate it.

There's formulae for everything. Now how to calculate the drag caused by something with the drag coefficient of that engine will require all the measurements and then you can calculate it. For this exact thing, maybe CFD is better


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How do flight controls work in tandem wing configuration?
If I understand it correctly, the usual configuration with tandem wings is that the front wing has ailerons - controls the roll. And the rear wing (a bit smaller than the front one) has elevators and rudder to control the pitch and yaw respectively. Am I right? Or are there any mistakes in my understanding?

Can you give us a bit of background? Are you designing an RC model? Why canard? Some particular characteristics you want to achieve?

For real tandem wings the rear wing is exactly the same size (not smaller) than the front. But yes, that's where you put the elevators. It's exactly the same logic for canards where the rear wings is also where you put the elevators (though they're normally configured as elevons - elevator+ailerons)

@sophit Yes, it will be an RC trainer. Though I decided to make the wings straight (picture updated), to make it easier to build.

Yes, I have. Though I am still trying to put the wings more apart = more stability, but I need my payload in front. Gotta find the sweet spot :).

I hav echoosen the flat bottom airfoil. But I am not sure why should my forward wing stall? As you can see on my image, the front wing generates lift and has ailerons. The rear one is sort of big lift generating stabilizer with elevators and a fin.

Thanks for replying. Its so big cause its a tandem wing. The design is kind of similar to the Switchbade 600 https://www.edrmagazine.eu/wp-content/uploads/2020/10/Switchblade600_03-1024x523.jpg. So for the plane to be aerodynamically correct, what would you suggest me to do with these wings. Thanks again for your time <3 .

There are many good answers here about tandem wing configuration, like this one or this one. Read especially carefully the comments of @PeterKämpf  P.s.: I clean up a bit my comments.

I think your assumption, that the rear wing is a little bit smaller than the on in front is not necessarily true.
There might be tandem wing planes with that constellation, but in some papers I found, the wing size differs between different models. The papers [1] and [2].
In addition some images depicting the mentioned:

In the case of the Miles M.35 Libellula, featured in the photographs above, the control surface arrangement was as follows: the front wing had flaps outboard and elevators (for pitch control) inboard.  The rear wing had flaps inboard and ailerons (for roll control) outboard.  Also, as can be clearly seen in the photos, the rear wing had vertical fins with rudders (for yaw control).  (From page 18 of first cited source.)
If the front and rear wings have appreciably different span, it is logical to put the ailerons (roll control surfaces) on whichever wing has the larger span, and the M.35 followed this practice.

Thank you very much for your answer, I will put the ailerons on the bigger one. So there is no problem in putting elevators on the smaller one in the image I have added to my question, I plan to use the front wing with ailerons and the rear wing with elevators. I think that is matching in what you have suggested. Thank you once again.

Many tandem-wing airplanes have a rudder (for yaw control) and vertical fin on the fuselage, not on the tips of the rear wing. The former is arguably more "usual" configuration. Examples: Rutan "Quickie", "Flying Flea".  See also the concept design pictured in the first image, and described in the second cited source, of this related answer to the present question.
On the "Quickie", the front surface wing had full-span control surfaces that functioned as elevators.  The rear wing had ailerons, mounted inboard.
On the (rather unusual) "Flying Flea", the entire front wing pivoted up and down for pitch control.  Roll control was accomplished solely by the rudder, via aerodynamic coupling between yaw (or more properly slip) and roll, thanks to the generous dihedral of the front wing, as well as the high-mounted position of the front wing.
With a tandem-wing aircraft, it makes intuitive sense that you would want the control surfaces to move in such a way as to immediately increase, not decrease, the lift of one of the surfaces to create a nose-up pitch torque, and this only works if the elevators (pitch control surfaces) are on the front wing.  However, many of the older "free-flight" model airplanes did have very large tails with lifting airfoils.  There have likely been some aircraft, in model form if not in full-scale form, that could arguably be described as "tandem wing" aircraft, with the rear "wing" not too much smaller than the front wing, that had the ailerons in the front and the elevators in the rear.  In general, it makes sense to put the ailerons on whichever wing has the greater span, although the inboard position of the ailerons on the Rutan "Quickie" seems to challenge this logic.

To "quiet flyer".......So, would you suggest to make the front wing smaller, or the rear wing smaller?
And as you have said  " In general, it makes sense to put the ailerons on whichever wing has the greater span", I will put ailerons on the big one and elevators on the smaller one.
I have edited my answer and added a picture of my plane, I will be glad to hear any suggestions, as you clearly aren't new to this topic :).
Thank you sincerely for your time.

@Mark -- I  need to be careful about getting involved in conversations in comments-- Stack Exchange policies discourage that.   If you gather 20 reputations points you'll be able to post in "chat" rooms and maybe we could discuss some more.  But honestly I don't think I have a great deal more to add-- I don't actually design airplanes, either full-scale or model...

The quickie's front surface was elevator only. The Rutan tandems didn't have flaps.

@JohnK -- that makes sense.  Apparently there is an error in the wikipedia article, in the section titled "configuration".  https://en.wikipedia.org/wiki/Rutan_Quickie#Configuration


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How does the RQ4-Global Hawk improve stability?
I have seen several videos recorded from an UAV in which the stabilization of the camera is almost perfect. Cameras are installed in a gimbal, of course, but do new drones introduce some new technology in order to improve the stability?
We can take, for example, the RQ-4 Global Hawk (first flight in 1998) and the Chendgu HQ (First flight in 2012). Note that the HQ-4 Xianglong's airframe topology its quite similar to the RQ-Global Haw, however it has a special wing configuration.

I count three questions in there, maybe more.

You might consider how many cell phones and digital cameras have some form of image stabilization these days: https://en.wikipedia.org/wiki/Image_stabilization

https://en.wikipedia.org/wiki/Synthetic-aperture_radar

The link is not directly related but I simply wanted to point out that in addition to gyroscopic and optical stabilization which answers your question digital processing allows modern systems to actually use movement of the platform to improve resolution. I doubt anyone actually does for cameras but it is possible.

@VilleNiemi what you mention sounds to me like image stacking, which yields great results for amateur astronomers using a widely-known algorithm.  I'm no expert, but I'd be amazed if military-grade drones aren't routinely using the technique also.

What you are looking for is "gyroscopic-stabilization".
As you can see in this video, the camera is mounted on gimbals that can rotate over all axis. Movements are controlled thanks to gyroscopes. For sensors mounted on larger aircraft, optronics systems are ball-shapes. That way they can rotate without modifying aerodynamics. There is no need to compensate for translation as targets are far away.
I didn't find any sources for shake reduction thus I can only assume techniques are the one that Wikipedia name "vibration reduction" and "anti-shake".

Also, flying at high altitude avoids the small-scale gusts which might cause shaking and changes the sighting angle only very slowly. The platforms themselves are almost stationary relative to the observed objects on the ground.


text:
How the maximum service interval is related to UAVs?
I´ve been reading about the aircraft´s maximum service interval but this concepts is hardly related to the day to day aircraft (civil and military ones of course). How does this concept is related to UAVs?
Note: I´ve restructured the question, I´m sorry for the previous one

Service interval  - is a maintenance term indicating the interval (either in time or number of flight cycles) between two successive services (e.g. inspection, change of oil, etc.) that a component requires.
In case of UAVs this is not different. It can be for example that the Service Interval of the engine is 100 hours of flight time, meaning that the UAV can fly missions for 100 Hours between engine maintenaince services.


text:
Why RNAV SIDs and STARs are RNAV 1 instead of RNP 1?
I am studying for my CFII check-ride and I was wondering to know why SIDs and STARs are not RNP 1 but RNAV 1?
Let's start with the following two definitions
RNP - Required Navigation Performance: It's a PBN navigation method/specification in which the airplane achieves an specified accuracy (RNP 1, RNP 2, etc) the a 95 % of the flight time. It has an onboard monitoring system that alerts the pilot when the integrity is loss. Examples of Equipments that are able to perform RNP 1, RNP2, etc are IFR GPS receivers with RAIM or WAAS (integrity monitor).
RNAV - Area Navigation: As we know, FAA uses RNAV in chart tittles only to determine Area Navigation. But RNAV it is also a PBN method/specification in which the airplane achieves an specified accuracy (RNAV 1, RNAV 2, etc) the a 95 % of the flight time. The only difference is that the aircraft won't alert the pilot when the integrity of the system is loss. I've never used an RNAV system without integrity monitor, I guess that DME/DME/IRU is wan of those. Is an IFR GPS with RAIM capability lost considered an RNAV system without integrity monitor?
The final and most important question is: On SIDs and STARs, Why is it not required to have a system that monitors the integrity of the equipment and alert the pilot if the accuracy can't be determined? Are pilots supposed to monitor the integrity of the systems by themselves? How?
Thanks
Edit
I found the following on AC 90-100A CHG2 8. RNAV SYSTEM APPROVAL PROCESS (b)(1)
"For procedures requiring GPS and/or aircraft approvals requiring GPS, if the navigation system
does not automatically alert the flightcrew of a loss of GPS, the operator must develop procedures to verify correct GPS operation"

Which SIDs and STARs specifically are you talking about? Not every SID/STAR in the world is RNAV ...

I am not talking about conventional Ground based NAVAID SID/STAR. So, not any RNAV SID/STAR in particular, but most of them are RNAV 1. I know there are some that requires RNP 1.

As far as I understand procedures are RNP if it's necessary for terrain separation and just RNAV if it's only for traffic flow (where controllers and TCAS can fix things up).

First of all, an RNP 1 procedure requires a GPS/ GNSS. And as you have said, it requires an inbuilt navigation monitoring and an alerting system. On the other hand, an RNAV 1 procedure does not. In RNAV 1 you can get navigational aid from the IRS/ VOR/ DME or IRS/ DME/ DME.
The SIDs and STARs can either be RNAV 1 or RNP 1. RNAV 1 is the system introduced at the start of performance based navigation to cater for smaller air spaces (terminal area procedures) and it is a step towards changing over to RNP 1. Many countries these days require an aircraft with RNP 1 capability to fly their SIDs and STARs. They sometimes tend to name it unambiguously on the chart. Like a chart might state, RNAV 1 and then state that you require a GNSS. This essentially makes the procedure an RNP 1 procedure. Others state specifically that you need to be RNP 1 complaint. This means that you need to have a GPS and a monitoring and alerting system. This is mainly seen in China, where most of the airports require the aircraft to be able to fly RNP 1 procedures so that they can pack airplanes together, with minimal ATS surveillance. This is done to accommodate the ever increasing air traffic in the region.
It has been quite a while, since I have not been to Wuhan, China for obvious reasons. Wuhan airport is one those example airports which allow only aircraft that can fly RNP 1 for their SIDs and STARs.

I think the advantage of keeping the charts and procedures RNAV is that it allows operation of older aircraft, which does not comply with RNP operations. This is I believe quite important in a place like the United States where airlines still tend to operate older aircraft and has a big general aviation sector. In other places, airlines tend to have newer aircraft, which easily fits into the current navigation standards. This makes it easy for the regulators to change their procedures to RNP.
For the question regarding losing RAIM (Receiver autonomous integrity monitoring). If you lose RAIM, you probably cannot do a GPS/ RNAV (GNSS)/ RNP approach (same type of approach, just different names). But the aircraft can still be used for RNAV navigation, as RNAV does not require the monitoring and alerting.

Like a chart might state, RNAV 1 and then state that you require a GNSS. This essentially makes the procedure an RNP 1 procedure.

Not correct. “RNAV1 – GNSS required” means that you can fly the procedure with RAIM inop (or with a non-RAIM capable receiver) but we, as designers, have only considered the GNSS scenario when calculating the navigation system error (NSE) component of total system error (TSE) for the procedure.
The reason for using RNAV1 as opposed to RNP1 is simply because it would have been considered quite adequate for the STAR or SID in question. Adding superfluous requirements could mean that an aircraft would have to abort the procedure if they temporarily lose RAIM capability (or get an alert), costing money for no added safety benefit.


text:
How to navigate onto VOR RWY 18 approach at KDEC?
Maybe a dumb question.  I've seen a few places where a VOR approach doesn't have an obvious (to me) way to get lined up inbound without radar vectors.
Say you don't have GPS and you're navigating with VORs.  You'd like to do the VOR RWY 18 approach to KDEC, and you're coming from somewhere north of the AXC VOR the approach is based on, like the BMI VOR for example.  How would you get yourself established on this approach without radar help?
It seems reasonable to me to fly (off-airway) R-180 outbound from BMI and intercept the approach course of R-349 inbound to AXC.  However I'd heard (though don't have any regs to back it up yet) you can only transition from enroute to an approach based on a fix that appears on both charts.  So I suppose you can't essentially vector yourself this way with the off-airway R-180.  Is this true?  If so, I'd be super grateful for a FAR/AIM reference if you know it!
So then what fix appears on both the enroute chart and the approach plate?  Only AXC is on both.  But if you fly directly to AXC from BMI then you're facing the wrong direction.  You'd be incoming on roughly R-352 and then need to turn around and fly outbound on R-349.  A HILPT would be just the thing on an approach, but here we wouldn't be on an actual approach yet and I haven't heard of them being used outside approaches.  Would you do a HILPT at that depicted hold, or just hang a uey here, or is neither really right?
As I said, maybe a dumb question, but it's stumping me.  Thanks for the help!

Approach plate added now.  I had it linked so it wouldn't be such a long post.  But it's easier this way, you're right :)

I would expect to receive radar vectors to final if I was coming from the north into Decatur, IL.   However, we do not plan on radar vectors when we flight plan because we could lose our ability to communicate with ATC.
I would fly the victor airways to AXC.  I might plan on coming in on the 029 radial (V191) to AXC.  I am hoping for a clearance for the approach by the time I get to AXC.  When I hit AXC, I would them make a right turn and proceed outbound on the 349 radial on the approach plate.
With a proper clearance, I would maintain my cleared altitude till TRACS and then descend to 3100 FT and finish the approach using the procedure turn to line up with the runway.
You are more than welcome to fly off routes provided you remain within the service volume of the navaid you are using.  For AXC the service volume is from 1000' AGL to 40NM (for low altitude flight).  There are no restrictions to the service volume in the Chart Supplements.  If you are going to do this (and ATC clears you), you will fly the OROCA as a minimum altitude.  The off-route obstacle clearance altitude (OROCA) for this quadrangle is 3200 FT MSL.  You could file BLM direct AXC and only be one degree off the final approach course.  (See AIM 5-4-7(f))
However, you are still required to cross an IAF before you start the approach.  You will fly over AXC and then turn around to fly the procedure turn... you cannot just cross TRACS and descend for the landing.
A procedure turn is always required if it is depicted on the chart with four exceptions (AIM 5-4-9)

Receiving radar vectors to final approach course
On a route marked "NoPT" on the approach plate
Received a clearance for a straight-in apporach from ATC
Performing a timed apporach

Thanks for your response.  Would you plan on the 029 radial inbound just to keep that turn to 349 outbound not as tight, or is there another reason?

Yes.   That is the reason.  You can also do a smart turn and not necessarily have to fly over AXC on the turn outbound.

I disagree rather strongly with "you will fly the OROCA as a minimum altitude". That's not what OROCAs are for. OROCAs, are advisory only, and are not good minima generally (think an approach on the plains in a quadrangle with a tall but far away mountain). AIM 5-4-7 does, however, say "ATC will, except when conducting a radar approach... assign an altitude to maintain until the aircraft is established on a segment of a published route or instrument approach procedure." The correct altitude to fly is the one assigned by ATC until reaching a published route or the IAF.


text:
How does a fan in a turbofan engine suck air in?
I watched lots of video related to sucking process of a fan in a turbofan engine, but I'm confused at a point.

In this link and many videos like this, I found the fan's direction wrong. This fan's blades in the video have high pressure on their top and right, low pressure on their bottom and left.
So in this case, air must go left, not right.

I think that these red dots are high pressure, yellow ones are low pressure since the fan pushes the air up by rotating upwards.
Can you explain this rotation direction and thrust to me?

"..the fan pushes the air up by rotating upwards". Remember Newton's third law? If the fan moves up, the air moves down! Think of an aircraft. The aircraft moves up by deflecting air downwards.

I agree but this is the problem. If the air pushes blades of the fan down, then it means thrust is to the left instead of the right in this geometry.

The HP air to the right tries to travel towards LP to the left, but it can't do so as the blades are in its way. In its attempt to travel to the left, the HP air pushes the blade to the left and that produces thrust. The air enters the intake due to the lower than ambient pressure to the left; air exits the exhaust due to the higher than ambient pressure to the right.

As are many questions that are rooted in an incomplete understanding of things, I think this is a great question that has prompted some very instructive answers.

I don't see any issue with the engine as depicted in the video. I don't think about pressure etc, but a rather simpler thought that if those blades, turning as depicted, were pushed into a giant puck of soft butter - would they lift butter off the surface and towards the engine's innards? And the answer is yes, they would. (I accept the limitations of this visualisation in that most turbofans turn faster than this animation, and are rarely used to gather butter.)

@Jawel7 Re "If the air pushes blades of the fan down, then it means thrust is to the left instead of the right" indeed the thrust is to the left. how else would the aircraft move forward?

Starting with the marine prop example you used in the comment on John's answer. It creates high pressure behind the prop and low pressure in front of the prop, so the BOAT moves forward. The water is accelerated backward. Think about the boat being held in place. The water would go out the back of the prop.
You have to think in terms not of the air that's at the blade itself but the free stream air. The low pressure at the front side of the blade causes free stream air to accelerate toward the blade. The high pressure at the back causes it to move away from the blade. The high pressure air directly at the blade can't move forward because the blade is there.
Think of just a plain wing. Low pressure on top, high on the bottom. It would LIKE to move from bottom to top, but the wing is in the way. At the tips it does just that, but the bigger force is the free stream air being pulled toward the top of the wing and accelerating downward. So the overall motion of the air is downward. The blade is just an airfoil situated sideways.

have high pressure on their top and right, low pressure on their bottom and left. So in this case, air must go left

Your observation is correct. The moving fan creates a high pressure zone behind it and a low pressure zone in front of it. However your conclusion is wrong. It is not a "must" that air go to the left.
Consider a balloon (just a regular one, made of rubber and inflated by human breath). The air pressure inside the balloon is higher than the air pressure outside the balloon. Therefore by your reasoning you may think that air must move from the inside of the balloon to the outside. But this does not happen. The balloon stays inflated. Why is that? It's because there is a wall separating the inside of the balloon from the outside - the rubber skin.
Exactly the same thing happens with the fan blade. The movement of the fan creates a zone of high pressure behind it and a zone of low pressure in front of it. But the high pressure air does not flow to the low pressure zone for exactly the same reason - there is a literal wall separating them: the fan blades.
But you say, "look: there are gaps between the blades"! Yes, and some air can move through the gaps. However air has mass and therefore inertia. Air cannot instantaneously teleport from one location to another. By the time the air tries to move into that open gap the fan blade would have moved to cover it (but obviously leaving a gap behind it - then the air may try to move into that gap but by the time it can get there the fan blade from behind will move to cover that gap (leaving a gap in front of it but again it takes time for air to move and by the time it can get to that gap the fan blade would advance to cover the gap... repeat to infinity)).
Now, if this is all that's happening sooner or later the two sides will sort of equalize anyway and there would be no pressure difference. All the fan would do is create turbulence in the local space with air bleeding through to the low pressure side via gaps we can't quite cover (eg. the gap between the fan tips and the engine's duct/body). You can see this phenomena happening in blenders. However, thinking this way ignores an important fact: the fan is not the only object in this universe - the fan is operating in a whole universe that surrounds it.
One thing you are missing from your thinking is that the entire engine is surrounded by the planet's atmosphere. Yes, there is a low pressure zone in front of the fan blades but you are forgetting that there is an atmospheric pressure zone in front of that zone. Since air moves from high pressure to low pressure and since the atmospheric pressure is higher than the zone in front of the blades air will move from the outside world into the engine. This is the sucking action.
It's the same for air behind the blades: yes there is a high pressure zone behind the blade but there is an atmospheric pressure zone behind that zone. So air will move form that high pressure zone to the low pressure of the atmosphere behind it. This is the blowing action.
But how do the blades themselves create the high pressure and low pressure zones? The same way aircraft wings do it (or your cupped hand do it when you put your hand out the window of a moving car). The aerodynamic effect of the blade's airfoil coupled with the blade's angle of attack creates the pressure differential. We normally call this effect "lift" but in fans we call it "thrust".
And yes, as I mentioned earlier this lift/thrust is a transient effect (you can think of it as the blade beating down on the air). If left alone the air would move back to their original positions and thus no net air movement would be created. But the blade moves continuously, constantly creating the pressure differential and not giving the air a chance to recover back to equilibrium. It's kind of like the difference between swinging your hand and putting your hand out of a moving car. When you swing your hand you do get lift but since you stop swinging at the end you don't even have time to feel that lift. Putting your hand out of a moving car however you can feel the lift because your hand is constantly moving into fresh air and thus never allowing the high pressure zone below your hand to equalize with the low pressure zone above your hand. It is the constant movement that forces the two zones to remain separate.
If the engine were to stop spinning you will find that the high pressure zone and low pressure zone will disappear. This may sound kind of obvious but they disappear not just because the engine is off. They disappear because once the fan blades stop moving the air from the high pressure zone can equalize with the low pressure zone because there is no more moving fan blades stopping it from doing so.

Amazingly detailed explanation and helpful to be honest. So let me summarize briefly;
1-) My analysis that fan blades create low pressure area in front of them, and high pressure area behind them. I said that air should intend to go out of the engine, instead of getting sucked in, which is wrong, since you say that yes there is low pressure area in front of the fan blades, but there's also another air zone which is high pressure zone relatively called "ambient". It basically means that air in the surrounding area will intend to go from higher pressure to lower pressure towards engine.

This is the sucking action actually. Force and intention of the air behind the fan blades to go out is less than force and intention of the air in the ambient to get in the engine. As results, air goes in, not goes out. This is also why a turbofan engine's big fan is located in a tube somewhere a bit deep, because we want blades to rotate as fast as possible by accelerating air molecules while decreasing pressure of air front, and increasing pressure of air behind. Nevertheless, there's still air outside with higher pressure than air inside in front of the blades, and this why air comes in.

Did I understand precisely? @slebetman

Basically yes. And as for the reason for the tube (and why the fan is in the tube instead of just outside).. Remember I mention that air can bleed from the high pressure at the blade tips? Have you ever seen pictures of airplanes flying through a cloud or some fog or mist? Do you notice that circular airflow from below the wings? That's the high pressure air attempting to flow to the low pressure region. We call this tip vortex. The tube is there to prevent this from happening so that we maintain more of that high pressure behind the blades...

... note however that there are pros and cons of having a fan inside a tube. While we do recover pressure loss from preventing tip vortex the tube itself is a source of drag. This is why regular propeller planes are still used in the real world instead of everyone converting to ducted fans - the weight of the tube plus the drag of the tube is enough to offset any advantages we gain from preventing the high pressure air from bleeding into the low pressure zone. The alternate solution to minimize the loss is to increase the size of the props which is why planes like the Tu95 have giant props

@Jawel7 You may be interested in the propfan concept. Where they run the fan blades outside without the tube. Google "propfan". The difference with normal propeller planes (eg. turboprop) is that propfans run their fans at turbofan speeds (turboprops are geared and spin their prop slower than the core jet engine). Without the tube the fans run at improved efficiency. However so far they haven't gained market acceptance due to excessive noise (the tube reduces noise partly by removing tip vortex). This is an area of active research by NASA and US aviation industry to meet future carbon goals.

The video and the fixed image are correct. The blades are moving up viewed from the left side of the engine in that particular video, and the LP side (top of the airfoil) is forward, the HP side (bottom of the airfoil) is aft, so the lift force is forward, a Newtonian reaction to the air being accelerated aft by the rotation of all those little wings. You may be perceiving the blade orientation backwards somehow in the videos, but they look correct to me.
The fan is just a fixed pitch propeller, little different from the wooden propeller on a Piper Cub, except lots and lots of blades, driven by a windmill in the exhaust of the engine instead of directly via a crank and piston, and with a shroud around it. But otherwise, more or less the same thing.

My problem is that this rotation direction creates lower pressure on left, higher pressure on right, in my opinion. If so, then air flow will go left, not right. Why do you think that placement is correct?

@Jawel7, you are correct that higher pressure is on the right, because it is a compressor.  As the air flows from left to right it is being compressed, which increases pressure.  If the blades suddenly stopped turning this higher pressure air might briefly reverse direction to equalize with ambient, although inflight ram air would probably just push it through.  But the compressor blades are powered, they are doing work to force air flow to the right., compressing it in the process.

@MichaelHall I understand but then the question is why air goes to right from left? As aeroplane stops, there is no air flow. After engines start, aeroplane stars to move too. It means that engines suck air in, but how? If you agree that high pressure is on right, and low pressure on left of the fan, then why should air flows from left to right?

Air is sucked in because the fan blades are spinning because they are powered by the turbine. There are plenty of questions here, (and other material on-line) that explain how turbine engines function.  A full explanation is out of scope for a comment.

@Jawel7 I think you need to go look at an ordinary, everyday fan. Look at the angle of the fan blades, predict based on your inference which way the air will blow, then turn the fan on and feel for yourself if your prediction was correct.

@HiddenWindshield I did it many times, this is why I am surprised by this example. https://www.youtube.com/watch?v=ON_irzFAU9c
You can take a look at this link and see what I mean. A marine propeller rotates and creates thrust. It creates high pressure area at the back of it, and low pressure area at the front of it, so it goes forward, perfectly understandable. The surface pushing air/water has more pressure than counter surface. It means that air flows towards low pressure region. Now, does it really make sense to you the video I shared in my post?

@Jawel7, your written English is very good, but from your profile it is apparently not your first language. Perhaps some concepts are lost in translation? Otherwise I'm just not seeing where the disconnect is and it's frustrating me that I can't explain it. My advice: Disregard any videos having to do with submarine propellers, (water doesn't compress) and search for an explanation of how gas turbine engines work in your native language. Hopefully that will help make the light come on.

@Jawel7 You're right, if the fan stopped spinning then the air would move the direction you say it would. However, the fan is spinning and thus constantly beating the air backwards. It acts more like a wall, from the perspective of the high pressure half, and that pushes the fan and thus the aircraft forwards. Imagine if the marine propeller were put on the nose of the submarine!

@Jawel7 Yes, the video you posted made perfect sense to me. Low pressure on front, high pressure on the back, just like a turbine. And just like a turbine, the blade is pushed/pulled forward. Also, just like on a turbine, the fluid (water or air) is pushed backward.

@MichaelHall "If the blades suddenly stopped turning this higher pressure air might briefly reverse direction to equalize with ambient" Yes, that's exactly what it would do. That's what a compressor surge is (well, except that the blades don't actually stop spinning... that would be quite violent given their momentum) and it does indeed happen sometimes when an engine is damaged or its airflow becomes disrupted to the point of stalling the compressor.

Whenever air is pushed, there is higher pressure created in the direction of the push. And this is what's happening at the fan and at propellers: they push air backwards.
Eventually, the air will be part of the atmosphere again, so where does it flow to? Not through the sidewalls of the intake. Also not back to where it came from, because there is still more air being pushed in, by a fan tightly fitting in the inlet duct. Because of the construction of the engine, it can only flow outwards on the aft side of the engine.
Consider the view from the air just after the fan: it cannot flow back to the fan, because there is higher pressure there, more air being pumped by the fan. There is a lovely low pressure atmosphere in the back of the engine, and that is where it will flow to.

This is a very common source of confusion. The cause is that you are looking at the pressure distribution on the blade surface in isolation; and not the associated fluid velocities that are inextricably tied to that pressure distribution.
The solution to the fluid flow equilibrium equations results in both an unbalanced pressure distribution on the rotor and cowl, as well as induced flow velocities in the airstream. These are co-dependent on the system geometry. Given a pressure distribution, you can determine what the total flow field is; or given the flow field, you can determine what the pressure distribution over all the surfaces is.
Since the object is to produce thrust to the left, it makes sense that the pressure difference across the blades pushes the blade to the left. And the necessary reaction of the fluid wake is to have greater momentum to the right.
So you can't look at the pressure distribution and say what the air will do in response to that pressure distribution. There is only one thing the air can be doing that is consistent with that pressure distribution, and you can be certain that it is doing it.

First of all a couple of corrections:
The turbofan that you see in the video is a "geared turbofan". In this kind of engines the fan is not directly connected to the same shaft of the turbine(s) but there's a gearbox between the two. This gives the fan a direction of rotation that is opposite the one of the low-pressure compressor and turbine stages. Don't get fooled by this effect.
In the video, the turbofan is in flight. Normal cruise speeds for a jetliner is Mach 0.80 something. This value is way too high for the fan and for the first stages of the compressor and this is why there's an inlet in front of it: the inlet has exactly the task of slowing down the airflow till some Mach 0.4. That means that the engine does not really actively suck air in but rather it passively ingests air.
A turbojet/fan actively sucks air in basically only when it's on ground at rest. The easiest way to see this suction is to go back to the roots and obviously it cannot be done if not with Prandtl himself (please jump to minute 1:18):

Here you see what happen when an airfoil (or a blade or a wing...) passes by (to match your turbofan video where the blades move downward, just rotate the screen so that also the Prandtl wing moves downward). From the video one can see that when the blade passes by, it makes the airflow move from the top to the belly behind it i.e., if it where a compressor's blade, from the front to the core of the compressor stage. This is the "suck in" effect you were looking for.
In the next two pictures I highlighted (in red) where the airflow was just after the passage of the airfoil and (in blue) where it moved to after some seconds:

Obviously in a fan/compressor just after the first blade another one comes and then another one and again and again: this multiple-passage multiplies the "suck in" of the airflow behind each blade.

You can watch this same effect with a simple desk or ceiling fan.
As a fan blade rotates, air "bunches up" in front of it, and bunched up (high pressure) air is higher pressure than other air around it.
The high pressure air can't escape from front of the engine, because there's an angled fan blade hovering over it. So it instead moves into the engine.
Another way to think of it is just imagine hitting a rubber ball with a paddle angled at 45 degrees upward. The rubber ball would travel up into the air. Basically, your turbofan blades are your racket, and air molecules are the ball.

I don't really think that racket-ball is a good analogy to use when explaining "how a turbofan sucks air" and other aerodynamic principles as such. The air is sucked due to low pressure at the intake, and exhausted due to high pressure at exhaust. A single ball cannot replicate the concept of "pressure", and so it cannot be used to illustrate those principles.

The racket-ball example explains in a very simple and good way what happen to the airflow when it encounters a wing... unfortunately it is correct only at hypersonic speed.


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Effect of turbulent flow on flight
What effect does turbulent flow (let’s say over a wing) have on flight?
Could I say it reduces flight performance like lift, increase drag?

Performance of? Engines? Structure? Max flight speed? On board comfort? Cabin crew? Captain? Overhead locker?

Hi, I meant like drag, lift, fuel consumption

Ok. Next question: Do you mean turbolent like in "turbolent boundary layer" or like in "gusts"?

We'll need to talk about two kinds of turbulence...
When the pilot says 'we are expecting some turbulence' and you feel the aircraft bouncing around somewhat violently -- that turbulence is caused by non-uniform winds (often with a vertical component) that the aircraft is flying through.  Since they are non-uniform (localized), the aircraft encounters them as an unsteady event -- an updraft for a moment - and then gone.  These unsteady, non-uniform winds cause the local angle of attack on the aircraft to change -- causing changes in the aerodynamic forces that then bump and shake the aircraft around.
Fortunately, these events are relatively quick.  They do not effect the aircraft's fuel consumption, maximum speed, or rate of climb in any significant way.  However, the aircraft structure must be designed for gusts and turbulence.  It is a contribution to the safety factors that go into structural design of the aircraft -- that in turn determines how much the structure weighs.  So, in a sense -- if you could guarantee no turbulence, you could make a lighter aircraft that would burn less fuel, etc.  It would also break the first time it encountered a gust.
The other kind of turbulence is what we mean when we discuss whether a flow is laminar or turbulent.  Laminar flow is parallel, uniform, smooth flow in a boundary layer around an aircraft.  Turbulent flow is chaotic flow with tiny eddies of random motion in the boundary layer of an aircraft.
A turbulent flow will have a higher skin friction coefficient and will have more drag on the same surface area than a laminar flow would have.
However, that turbulent flow will tend to stay attached a little more than a laminar flow would in the same situation.  So, if the local drag is dominated by separation (not skin friction), a turbulent flow may have less drag than a laminar flow.
The latter is why golf balls have dimples.  The dimples encourage transition to turbulence (they 'trip' the boundary layer).  This delays separation and reduces the drag on the golf ball -- skin friction drag goes up, but the separation drag goes down more than enough to counter that.
This delay in separation will also tend to allow turbulent airfoils to reach a higher CLmax than the same airfoil in a laminar flow.
Most aircraft fly mainly in turbulent flow.  We design them expecting turbulent flow, so we don't think of it as reducing flight performance -- as we planned for it from the start.
High performance sailplanes are able to take advantage of significant laminar flow.  Their performance is negatively affected when dead bugs accumulate on the leading edge of the wing -- this can cause the boundary layer to trip and for the flow characteristics to change.  Some aggressive laminar flow airfoils have even demonstrated being sensitive to rain.


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When is it required to enter an airway? (IFR)
Assuming I'm not using a computer program to generate a flight plan for me, for cross-country IFR flight planning in the United States, how quickly after finishing departure procedures is one supposed to get into an airway?  To generalize the question a bit, I'm trying to understand when using an airway is required and when it's ok to route to waypoints outside of airways.
As a sim hobbyist one of the things I enjoy is learning about charting and route planning.
UPDATE: In addition to answers provided here, I also found it helpful to read Instrument Procedures Handbook, FAA, Ch 2, En Route Operations.

It's required when clearance delivery says "full route clearance, advise ready to copy..." ;)

Usually after a SID, you are at a waypoint along the route you want to fly.  If the SID ends with a radar vector, it is up to ATC to vector you to the airway they want you on.

There's no general requirement for an IFR aircraft to use airways at all, regardless of if you're using a computer to generate the route or not. They only exist to make planning easier.
Of course, if clearance delivery amends your flight plan with an airway, you're required to use it. Then again, they could just as easily give you a route that doesn't include an airway when you had one in your flight plan.


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Why wouldn't a plane start its take-off run from the very beginning of the runway to keep the option to utilize the full runway if necessary?
I was just watching BSL/LFSB on Flightradar24 and noticed that flight W67772 didn't start its take-off run from the end of the runway.
Wouldn't it make sense to start every take-off from the end to have more runway available in case you need it (for whatever reason)?

@sehe I recommend reading the answer that tedioustortoise has written now. It contains all and much more than what was in their original comment that I deleted.

Short answer:
Operational efficiency - significantly, you have saved time, and you might also save fuel from an unnecessary longer taxi.

Long Answer:
1. Operational Efficiency:
As discussed above, taking off from an intersection saves taxiing time. If you save this on two occasions during the day, you have saved 10 minutes. This might even mean the difference between missing an ATC slot restriction on your third sector later in the day. Remember, many major short-haul carriers seek to turn around their aircraft in less than half an hour. Minutes matter.
2. The takeoff is still completely safe from the intersection:
The aircraft in your question took off from the D intersection at LFSB/BSL.
From Intersection D: 2900m/9514 ft Takeoff run available (TORA)
Full RWY 33: 3900m/12795ft TORA
Here are some performance calculations for a 737-800 (reasonably analogous to the aircraft in your question), loaded with (a typical sort of flight) - 7 tonnes of fuel, 175 adult pax on board, and 700kg of baggage in Hold 2. The performance figures are from the approved source for LFSB/BSL: wind calm, 15 degrees Celsius outside air temperature, dry runway, and 1013 hPa QNH.

Intersection D: De-rate to 24k, using assumed temperature thrust reduction (known as Flex for other types) of 41 degrees C, v1 140 kts, Vr 141 kts, V2, 145kts
Full RWY 33: De-rate to 24k, using assumed temperature reduction (known as Flex for other types) of 41 degrees C, v1 140 kts, Vr 141 kts, V2, 145kts

From the data above, you can see that the aircraft will be set up identically for takeoff from runway intersection D or if it were to take off from the full runway.
So, in this instance, the further taxi from intersection D to the full runway would be around 1km (cf. airport charts) or perhaps around 120kg of fuel burnt. This may not sound like much, but if that airline operated 2000 flights per day, then it is a saving of almost 250 tonnes of fuel - costing perhaps around 200,000 $US, in just 1 day.
Please note that there may be circumstances where higher (or "less" de-rated) performance is required when the takeoff run is shorter from an intersection, which would potentially lead to higher fuel burn on the takeoff roll or more wear on the engines.
3. Takeoff safety for jet aircraft is arguably less about runway distance available and more about critical decision making around speeds close to V1.
Taking off from intersection D gives you a takeoff run available (TORA) of 2900m. To put that into perspective, the TORA at Bergamo runway full length is 2874m, TORA in Faro full length is 2490m, TORA at London Luton, both runways full length is 2162m (and on a hill), or finally at Bristol, where the TORA is 2011m (and on a hill). All these airports have a TORA less than that available from the D intersection at LFSB/BSL and are frequently used by jet aircraft without any concerns over safety.
At v1 speed of 140 knots, a 737 would travel the distance between the D intersection and the full runway length (1000m) in 13.89 seconds. (And 1000m is a large difference in length between intersections). Perhaps a more average distance between intersections is 400m, which gives crew another 5.56 seconds of decision time (really not much)…
Arguably what is therefore more critical from a safety perspective is not necessarily the increased runway available, but rather the decision making skills of the flying crew. If the aircraft was unsafe to takeoff the crew need to decisely act before v1 to reject the takeoff.
Furthermore, if the worst eventually happens (aside from a dual engine loss at v1), such as a major engine damage or fire at v1, the aircraft is still safe to fly - and flying crews regularly train for this scenario. It can actually be safer to get the aircraft into the air, secure the engine and return as opposed to stopping or trying to immediately land it back onto the runway.

As an airline pilot working for a major European airline I can testify that we regularly aim/plan to take off from intersections. We run the performance figures for intersections and will typically request from ATC to use an intersection if the cabin is secure for takeoff. (It is often the lack of cabin secure which prevents us from taking a planned intersection - cabin not ready so we “might as well continue” to the next intersection/runway endpoint).
Your question asks,

Wouldn't it make sense…to have more runway available

Safety is priority, however operational efficiency is also a priority.
The takeoff is safe from the intersection: now we think about saving time and fuel.

41 degrees c might be a bit high estimate for basel in May, but great answer. I appreciate that a high temperature estimate will only mean you get better performance at a lower realistic temperature - do you really high estimate on purpose?

@Jamiec 41 degrees is the assumed temperature under the assumed temperature thrust reduction method for the engines (Can be known as Flex for other types). The Outside air temperature used is 15 degrees in Basel. Cf. http://www.b737.org.uk/assumedtemp.htm

cool. I learned something else new today!

Wait, they burn 120 kg of fuel for 1 km of taxi ? That's approx 1.5% of the 7 tons you took as an estimate. What an amazing figure.

@PhilippImhof: Yeah, jet engines are more or less constant thrust, not constant power, over a range of speeds, and power is force * speed, so the amount of useful power per fuel used at taxi speeds is very low, and the total fuel burn adds up to significant greenhouse gas emissions.  (Pushing on the ground instead of a large amount of air would be way more efficient, that's why cars power the wheels).

That's why people are working on ideas like Has there been any attempt to use an underground cable system, a la a cable car, to tow aircraft along taxiways? (no, but yes for wheeled electric "tugs" that run on rails), and  Does it make sense towing airplanes to the head of airstrip by external (eg electric) means? mentions some other ideas like an electric motor in the landing gear.

As an example:
I did my flight training at Boeing Field (KBFI), which has two runways: a GA runway that is 3,700 feet long, and a commercial runway over 10,000 feet.
My flight school was near the middle of the commercial runway, so we frequently would taxi to intersection A9, and have nearly 5,000 feet of runway in front of us, whether taking off to the North or South.  (Note that 1/2 of the Commercial Runway is still more than the full length of the GA runway, and also closer to the flight school parking area)
5,000 feet is a LOT of space for a training Cessna.  The time and cost involved in taxiing to the full end of the runway was considerable, but really provided very little extra safety margin.  5,000 is already enough to recognize a bad take off and safely reject it, especially since most flights had an experienced CFI on board.
This is one clear case where it definitely makes sense to take off from the middle of the runway rather than use the full length.

Another example is at Johnson County Executive (KOJC) in Kansas, where taxiway Bravo goes almost, but not quite, to the end of runway 36.
To get the true end of the runway involves crossing the runway (with Tower clearance), and taxiing a bit further on taxiway Alpha.  That is extra complexity and ground driving, to get an extra 400 feet (approx) of runway.
A lot of planes choose to just request an intersection departure from "Bravo at Juliet" rather than "Cross 36 at Juliet, Join Alpha, Continue Taxi to 36".  They'll still have roughly 3,600 feet in front of them, which is still plenty of space for a safe takeoff or rejection.

So that's why they casually send trainers to B4 to go south.

by way of comparison, my home field has about 1,900ft - so yeah 5,000ft is a huge amount.

Yes, that makes sense of course. :) What you're describing makes sense for the training Cessna: the runway length is way oversized for its requirements, if I understand you correctly. But are city airports (like BSL/LFSB from my example) just as oversized for jets, too? I looked up the airport map and by starting take-off from that intersection, runway length was reduced from about 3.9 to 2.9 km. So where could I find out the required distance for an A321 which aborts take off juuuust before V1?

There's nothing more useless than fuel you left on the ground, or runway behind you.
We're generally taught to use all the runway, as you point out; why wouldn't you? What if you need it. However for the sake of expediency pilots may plan for, and request, take off from the intersection of the taxiway and the runway.
Flight W67772 looks like it was operated by an A321 from Basel. One must assume that there was plenty of runway available, including whatever contingency was required by Wizzair's SOP, to take of from that intersection.

Hmm, can you explain who would ask for an "intersection takeoff" and why? Is this basically a case of "We'd either take off long before runway ends or decide to abort and even then we'll still have more than enough runway left even if we don't start take-off from the very beginning"? Is the BSL runway that long compared to other airports or is this usual practice at all airports?

@CannedMeatLover is that clearer?

@Jamiec, the other useless thing is a half a second ago!

@Jamiec Yes, thank you. So basically ATC was asking them if it would be possible for them to "hurry up"? By starting take-off from that intersection, runway length was reduced from about 3.9 to 2.9 km. If I'm comparing that to other (small) airports, it seems like that's still a good amount of runway length. Where can I find out the required distance for an A321 which aborts take off juuuust before V1?

@CannedMeatLover there's no one number for take of run required (TORA). It will vary by surface, conditions and weight.

Yes, you're right. :) But there are minimums/maximums and average values... So you can get a rough idea, right? And also there must be an algorithm/table to calculate/look it up very precisely. I would assume this information is some kind of public aircraft manual?

@Jamiec - As an airline pilot working for a major European airline I can testify that we regularly plan to take off from intersections. The premise of your answer assumes that it is ATC prompting crews to use intersections which is simply not true - at least not in my experience across Europe. We run the performance figures for intersections and will request from ATC to use an intersection if the cabin is secure for takeoff. The goal is operational efficiency - time and fuel burn. We have already proven that the takeoff is safe from a performance perspective.

@CannedMeatLover v1 isn't a set speed. It's decided based on the conditions and how much runway you have available. The longer the runway, the higher you can make v1 without risking a runway overrun.

@tedioustortoise thanks for the insight, ive updated my answer. Flying light singles we sometimes get asked at larger airfields whether we can "accept an intersection departure" and I just assumed that was the same for the big guys. Again I sort of assumed pilots would have run the numbers knowing full well the likeliness of that request.

Clearly, the answer is yes. The OP asks, at the ending of their question -

Wouldn't it make sense to start every take-off from the end to have more runway available in case you need it (for whatever reason)?

This answer was offered previously regarding a similar consideration to the posted question.  The gist of that answer, which is relevant here, was summarized in the last paragraph of the introductory to the NTSB accident report, which is as follows...

The circumstances of the accident are consistent with the pilot failing to maintain sufficient airspeed following a loss of engine power during takeoff, resulting in the airplane's wing exceeding its critical angle-of-attack and a subsequent aerodynamic stall. Instead of using the full runway length of 6,179 ft, the pilot elected an intersection takeoff with about 2,570 ft of available runway. Calculations showed that, had the pilot used the entire runway for takeoff, sufficient runway for a landing following the loss of engine power would likely have been available.

Nevertheless, I am reminded by an executive pilot I know who has 12k+/- logged flight hours, "Tom, if the available runway is 17 thousand feet long, and only 1700 feet or a bit less is needed at full power to become airborne at the current density altitude, then a midfield departure using 8500 feet of runway is in order rather than a taxi all the way to the end of the runway, given all other things considered."  In that case the answer is no.  Hence the relevance to the answer provided by Jamiec, there is no need to unnecessarily consume fuel on the ground given adequate available runway for a midfield departure. Specifically, the issue here must be viewed in regard to the situation faced by the pilot, and the resources available to the pilot upon takeoff.  Hence, "why wouldn't" depends conditions at the time, namely meteorological conditions, physical availability of runway, and performance of the aircraft.

The question asked (in the title) is "Why wouldn't..." - not a yes/no question, while the question asked within the OP is "wouldn't it make sense to start every takeoff from the end...". Given your (excellent) example of the 17,000' runway case, the answer you write seems to contradict your first sentence.

Ah, I see your direction.  The offered edit should clarify the "Why wouldn't..." qualifier.  No problem...


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Why do the arresting gear cables need to be pushed/ guided back?
Also, what is the yellow metal stick officially called?

Do you have a source for the image? That may even be good start to finding an answer.

That’s done principally to make sure that the cross deck pendant does not get snagged on anything during the retraction process or pose a hazard to personnel nearby.

Most likely, it is to avoid this kind of situation:

Retraction sometimes causes formation of long, loose helical loops in the deck pendant. Such loops are usually eliminated as tension increases on the pendant. Occasionally a loop, instead of being eliminated, tightens into a small loop called a kink. Kinks can also form if the retraction cycle is halted abruptly; if the retraction is not a smooth, continuous operation; or if the torque is allowed to build up in the cable.

Additionally, the cable must be visually checked every time is used (if time allows it).
I have no idea about the name of the stick :)
Source: https://www.navair.navy.mil/sites/g/files/jejdrs536/files/2018-11/14353A_ch9.pdf


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Slanted Brown Rectangles on Aircraft Carriers?
What are the rectangles adjacent to the arresting gear on Aircraft Carriers? Are they service hatches?
https://www.kastalon.com/wp-content/uploads/2017/10/ford-emals-3.jpg

I might suggest you add the photo to your question in case the link breaks later.

The rectangles are called impact pads. The ends of the arresting cable tend to snap back onto the deck after an aircraft catches it. The replaceable polyurethane pads protect the carrier deck from the cables.

Source: NAVEDTRA 14310
As Michael Hall points out, the section of cable across the deck is called the cross deck pendant. Since it takes such a beating, it has to be replaced every 125 arrests. The rest of the cable goes below deck into the arresting engine that uses a combination of hydraulics and pneumatics to absorb the energy. This is the purchase cable and only needs to be replaced after around 1400 arrests.
It's much faster and cheaper to just replace the cross deck pendant, so it's attached to the purchase cable with terminal fittings. These heavy fittings are what can cause the most damage and the pads are located where they most often strike the deck.

Source: NAVEDTRA 14310

When you say "after the aircraft catches it" do you mean after the aircraft raises the hook and the cable snaps back or as the aircraft lands, the cable snaps back?

@aerojetman62 it's as the aircraft lands and catches the cable. After it stops and raises the hook the cable is retracted slowly.

Good edit.  Feel free to steal my photo and I might just delete my answer as yours is more complete.

Good answer by fooot already, but to add some detail, it isn't the cables themselves as much as this beefy steel "knuckle" connecting the more readily replacable cross-deck pendant to the much longer below deck cables that run through the sheaves.
Each trap is like cracking a whip, sending a ripple outwards from the hook point, that hammers this thing into the steel deck with quite a bit of force.  The replaceable impact pads reduce permanent damage to the underlying structure, and slightly dampen the harshness of the noise below decks.

Yes, I could see how that could cause quite a bit of erosion and damage to the flight deck after an extended period at sea.  I guess the pre-existing flight deck coating that is applied prior to deployment and periodically replaced is insufficient in those areas.

Neat point about noise below decks. If it's not too hard, could you drop in a link of the sound of these cables slapping onto the pads?

I’m afraid I didn’t make any recordings…